Reactor refueling method

ABSTRACT

A nuclear reactor is disclosed wherein hydraulic drive mechanisms are provided to vary the location of a plurality of neutron absorbing control elements associated with selected fuel assemblies. Each fuel assembly comprises a fixed array of commingled and spaced fuel pins and guide tubes. Each selected assembly has a plurality of individually movable two-position control elements which are coupled to drive mechanisms which operate to telescope the control elements within their associated guide tubes. The control elements are situated such that they can be withdrawn into and removed with the upper support structure for refueling.

United States Patent Frisch et al.

[451 Nov. 27, 1973 REACTOR REFUELING METHOD [75] Inventors: Erling Frisch, Pittsburgh; Harry N.

Andrews, Export, both of Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: July 8, 1970 [21] Appl. N0.: 53,206

Related U.S. Application Data [63] Continuation-impart of Ser. No. 700,121, Jan. 24,

1968, Pat. No. 3,607,629.

[52] US. Cl. 176/30, 176/86 L, 214/18 N [51] Int. Cl. G2lc 19/00 [58] Field of Search 176/30, 31, 32, 35, 176/36, 78, 86 L; 214/18 N [56] References Cited UNITED STATES PATENTS 3,282,793 11/1966 Jamrog 176/30 X 3,276,965 10/1966 Leyse 176/22 3,127,323 3/1964 Bray et a]. 176/30 X 3,143,479 8/1964 Hargo et a]. 176/30 3,293,136 12/1966 Harris 214/18 N 3,494,827 2/1970 Zinn 3,519,535 7/1970 French et al 176/78 X Hepps [5 7] ABSTRACT A nuclear reactor is disclosed wherein hydraulic drive mechanisms are provided to vary the location of a plurality of neutron absorbing control elements associated with selected fuel assemblies. Each fuel assembly comprises a fixed array of commingled and spaced fuel pins and guide tubes. Each selected assembly has a plurality of individually movable two-position control elements which are coupled to drive mechanisms which operate to telescope the control elements within their associated guide tubes. The control elements are situated such that they can be withdrawn into and removed with the upper support structure for refueling.

5 Claims, 14 Drawing Figures PATENTEBHUYRY I975 3.775. 246 sum 3 [)F 6 PAIENH-jnuuvm 1975 3.775246 REACTOR STEAM GEN.

MAIN COOLANT PUMP FIG. l3.

PATENTEI] NOV 27 I973 SHEET 6 BF 6 REACTOR REFUELING METHOD CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application 700,121 now U.S. Pat. No. 3,607,629 filed Jan. 24, 1968 by E. Frisch and H. N. Andrews, and assigned to the same assignee.

BACKGROUND OF THE INVENTION This invention pertains to the refueling of nuclear reactors, and more particularly to a method wherein the control elements can be completely withdrawn from the reactor pressure vessel to fully expose the nuclear core for refueling.

In accordance with the present state of the art, certain power reactors are controlled by a combination of chemical shim system and rod cluster control (RCC) system. The RCC system generally comprises 16 to 20 control or neutron absorbing modules adapted to be telescoped within cooperating guide tubes in selected fuel assemblies. All of the modules associated with each of the selected fuel assemblies are attached to a single spider which is in turn attached to a common drive shaft. Thus, all of the modules are operated simultaneously in discrete steps over the entire distance of travel.

As indicated and explained in detail in copending application Ser. No. 700,153, filed of even date by R. French, and H. Andrews for a Nuclear Reactor, and assigned to the present assignee, reactors utilizing RCC control or other incrementally movable control elements have several limiting characteristics. The worth of each control cluster is too great to be used for suppressing radial flux peaks. Partial insertion of a cluster can cause severe perturbations in the axial flux distribution and can lead to xenon cycling.

An optimum control system would accordingly have two primary characteristics. A wide dispersal of individually movable low worth absorber modules, and no control configuration wherein certain control modules are partially inserted. Such a control system would result in appreciable savings due to more efficient usage of nuclear fuel.

A highly reliable drive mechanism which is capable of positioning a plurality of two-position control elements is necessary to render such a desirable control system practical. Previous attempts to design and utilize multi-element drive mechanism had many obstacles to overcome. Large reactors which utilize a wide dispersal of control modules might contain 1,000 or more such modules. This means that 100 or more mechanisms capable of positioning a plurality of modules, in groups of or more in this example, would be necessary. The individual mechanisms must not be so large that they cannot be mounted side by side on the reactor. Moreover, where attempts have been made to utilize the pressure inherent in many reactor systems to drive the control modules so as to efiectuate size reductions, the amount of fluid flow necessary to hold a majority, say 600, of the modules in the withdrawn position was found to require prohibitively large pumps. Recently, it has been found that electromagnets are an effective and practical means for holding the modules in their withdrawn position. Electromagnets are also utilized to operate the valves for such a mechanism. The problem with placing a plurality of electromagnets It is accordingly an object of this invention to provide a control system which enables a nuclear reactor to be refueled with the control elements removed.

SUMMARY OF THE INVENTION The aforementioned requirements are met by a hydraulic mechanism which utilizes the substantial pressure available in PWR or other pressurized reactors to move the absorber modules relative to the stationary fuel assemblies with which they are associated. The pressure housing of each hydraulic mechanism is mounted on and through the head of the reactor so as to form a leak tight seal. Interior of the pressure housing are a plurality of conduits or cylinders, each of which has a piston associated therewith. Drive rods are attached to the pistons and extend into the core region r of the reactor. Each drive shaft has one or more absorber modules attached to it for telescoping movement into the guide tubes of stationary fuel assemblies. A variable source of pressure is associated 'with the cylinders to accomplish a reduction in pressure and allow the pistons to move into the cylinder due to the high constant pressure on the opposite side of each piston, or to increase the pressure to assist in scramming the neutron absorbing control elements.

A support tube is mounted exterior of each pressure housing and is slidably engaged thereto. The support tube has a number of electromagnets mounted on its outward surface. One or more such electromagnets are associated with each cylinder at approximately the outward limit of travel of the associated pistons. Actuation of selected electromagnets holds the associated pistons at their outward or withdrawn position so that a differential pressure is not required to hold each withdrawn piston in this position.

Withdrawal of a control element, i.e., a number of absorber modules affixed to a single drive rod by a spider attachment, from its associated fuel assemblies is accomplished by utilizing the variable source of pressure to lower the force on the outward surface of the piston. This allows the higher force due to the pressure inherent in an operating PWR reactor to push the piston and its associated control element outwardly.

Selected control elements are inserted in the core by deactivating their associated electromagnets and equalizing the pressure outward of the piston. This then permits the control elements, attached to pistons which are not magnetically held, to be inserted by gravity into their associated fuel assemblies.

The elements which are most likely to fail during opmoved from the pressure housing. The upper extremities of the cylinders are also in communication with the variable pressure line so as to remove any gas bubbles which would reduce damping of the upward moving pistons at the end of their withdrawal.

The combination of the above enumerated features constitutes a mechanism which allows a reactor to be controlled by a large and diverse pattern of low worth two-position control elements and thus to more nearly achieve the full potential of its fissionable fuel. In accordance with the invention, a number of drive means each connected to a relatively small number of control modules are movable individually and independently of each other by a single control mechanism as contrasted with mechanisms of the prior art wherein each mechanism operates only one drive means. It is a further characteristic of the control mechanism of this invention that the control elements can be withdrawn into the upper structure and removed therewith prior to refueling.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention reference may be had to the drawings, in which:

FIG. 1 is a sectional view of a portion of a nuclear reactor having widely dispersed individually movable two-position control elements;

FIGS. 2, 3 and 4, taken end to end, show the drive mechanism of this invention wherein a piston is shown in FIG. 3 and in phantom in FIG. 4;

FIG. 5 is a cross-sectional view taken along line V-V of FIG. 2;

FIG. 6 is a view taken along line VIVI of FIG. 5;

FIG. 7 is a cross-sectional view taken along line VII- -VII of FIG. 2;

FIG. 8 is a cross-sectional view taken along line VIII- VIII of FIG. 3;

FIG. 9 is a cross-sectional view taken along line IX--IX of FIG. 3;

FIG. 10 is a cross-sectional view taken along line X--X of FIG. 3;

FIG. 11 is a cross-sectional view taken along line XIXI of FIG. 4;

FIG. 12 is a cross-sectional view taken along line XII- XII of FIG. 4;

FIG. 13 shows a schematic view of a pressure system for driving control elements; and

FIG. 14 is a view of the nuclear reactor being prepared for refueling and wherein the control elements are held in their withdrawn position just before all of the internals are removed to expose the fuel assemblies.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is illustrated a nuclear reactor incorporating the hydraulic drive mechanism of this invention. A pressure vessel 10 is shown which forms a tight pressurized container when sealed by head assembly 12. The pressure vessel 10 has coolant flow inlet nozzles (not shown) and coolant out flow nozzles 16 in and through the cylindrical wall thereof. The head assembly 12 has a plurality of head penetration adaptors l8 sealed in and through its substantially hemispherical wall. The axis of each head penetration adaptor 18 is aligned parallel with the axis of the pressure vessel 10. A core barrel 20 is supportedly suspended from an inwardly extending projection 21 just below the top of the pressure vessel 10. An upper support plate 22 and an upper core plate 24 joined by columns 23 are substantially supported by the core barrel 20. A lower core plate (not shown) is also supported by the core barrel 20. Coolant flow entering through the inlet nozzles (not shown) proceeds to a bottom coolant manifold (not shown) through a flow annulus 30 formed by the lower wall of the pressure vessel 10 and the core barrel 20 as is well known in the art. The flow annulus 30 also contains a radiation shield 32. From the bottom coolant manifold, the flow proceeds generally axially upward through the fuel assemblies 36 supported between the core plates to an upper manifold region 34 from which it then proceeds through outlet nozzles 16 to energy conversion structure (not shown).

The parallel array of fuel assemblies 36 include a plurality of fuel rods 38 and a plurality of guide tubes 40 symmetrically interspersed therebetween, and held in a fixed relationship each to the other by an egg crate type grid structure 41. This type of so-called canless fuel assembly is shown and described in a copending application Ser. No. 522,976, filed on May 25, I964 by R. Creagan and E. Frisch for a fuel assembly for nuclear reactors, now continuation application Ser. No. 721,122, filed Apr. 12, 1968, and assigned to the present assignee. The guide tubes 40 serve as receptacles for absorber modules 42 as contemplated by this invention. For purposes of description sixteen absorber modules 42 are shown per assembly (see FIG. 5), but this number may be changed to suit conditions.

The worth of a single absorber module 42 is so calibrated that the insertion of one or two such modules will not greatly change the power distribution of the entire core as would the insertion, for example, of a complete rod cluster control group. However, a single module 42 would have enough worth to reduce the flux density in its immediate environs.

The location of absorber modules 42 relative to fuel assemblies 36 is shown in FIG. 5. The absorber modules 42 are joined in pairs at their upper ends to thin gusset plates 44 to which drive shafts 46 are also attached as can be seen more clearly in FIG. 6. As will be explained, each drive shaft 46 is designed for individual movement. Although two absorber modules 42 are shown attached to each drive shaft 46, only one might be used, or in the alternative more than two might be used to form each control element 48. Control elements 48 are not employed in all of the fuel assemblies 36 in this example. However, all the fuel assemblies 36 are identical and have the same number of guide tubes 40. The guide tubes for the fuel assemblies 36, which do not have control elements 48 associated therewith, may be plugged at the top to prevent coolant loss through the tubes.

In this embodiment, eight drive shafts 46 are associated with each selected fuel assembly 36 and are arranged in a circle about the center line of each assembly 36 and extend vertically upward into a drive mechanism 50 mounted on the reactor head assembly penetration adaptor 18 on the same center line. Each mechanism 50 is designed to provide independent operation of the eight associated drive shafts 46 and their associated control elements 48.

As may be seen in FIG. 3, the drive shafts 46 terminate in drive pistons 52 at their upper ends which operate in long cylindrical structures interior of the mechanism 50, as will be described. The pistons 52 are moved in the upward direction by hydraulic force using the coolant water as the hydraulic fluid and the inherent a pump or other suitable means outside of the reactor] as will be described.

Drive shafts 46 are supported above the upper support plate 22 by a flange 56 to which is attached a vertical central shaft 58. Secured to the support shaft 58 by welding are several circular guide plates 60, spaced at regular intervals along the shaft 58, and a guide block 62 at the upper end. The guide plates 60 and the guide block 62 are provided with holes 64 and 66 respectively, to support and guide the drive shaft 46. It may be noted that in order to facilitate assembly of the reactor structure that funnels 68 are provided to slide over the central support shaft 58 and the guide plates 60 which is supported thereon. Radial slots 71 are also provided in the guide block 62 and the guide plates 60 to facilitate assembly (see FIG. 7). The central support shaft 58 possesses sufficient flexibility to compensate for any anticipated misalignment.

Resting on top of the fixed guide block 62 supported on central support shaft 58 is a cylindrical guide block lifting fixture 70 which is located on guide block 62 by means of a central pin 72 with a pointed end. The lifting fixture 70 is provided with eight vertical holes 74 with ledges 76 into which the drive shaft pistons 52 nest when the control rods are in their full inserted position. The pistons 52 in FIG. 3 are fractionally above their nested position. Any anticipated misalignment between the pistons 52 and their guide channels will be compensated for automatically by a slight rotation of the guide block lifting fixture 70 on its pointed pin 72. Because of the flexibility of the drive shafts 46 this last mentioned feature will obviate any jamming. It is to be noted that guide block lifting fixture 70 has a ballshaped upper extension 78 attached thereto. The ballshaped extension 78 may be grasped by a special lifting tool so that the captured pistons can be withdrawn from the reactor as will be later explained.

The above delineated structure should be viewed as part of the reactor and its internals as will appear clear from a further study. Turning now to the drive mechanism 50, the drive mechanism housing comprises a cylindrical base 80 having an outwardly extending flange 82 at its lower extremity and an upper cylindrical head cap 84 joined by welding to an intermediate heavy walled tube 86. Cylinders for the drive rod pistons 52 are formed in the base 80 and the head cap 84 by drilling axial holes 88 and 90 respectively therein. Cylinders are provided interior of the intermediate tube 86 by long tubes 92 which are fixedly supported in tight counterbores in the opening of the holes 88 and 90. Between the counterbore supporting points, the long tubes 92 are anchored in guide disks 94. The guide disks are in turn attached by welding or other suitable means to a central shaft 96 suspended between the base 80 and the cap 84. At the upper end, the mechanism interior is sealed off to form a pressurized housing by a threaded and seal welded plug 98 or other suitable means.

The drive mechanism housing is attached to studs 100 to the outwardly extending upper flange 102 of head penetration adaptors 18 and hermetically sealed by a high pressure gasket 104. Accurate radial positioning is obtained by dowel pins 106. The central space 108 directly below the mechanism which is open to the pressure vessel interior carries a pressure in this example of approximately 2000 psi. This space is separated from a surrounding annular space 110 having a pressure in this example of approximately 1700 psi by means of a tubular darn 112 and a metal O-ring gasket 114. In order to withdraw the control rod drive shafts 46 from the stationary fuel assemblies 36, the pressure in the cylinders above the drive pistons 52 must be lowered by connecting them hydraulically to the low pressure annulus 110. This is accomplished by energizing the coil 1 16 of magnetic valve 1 18 thereby lifting valve stem and tip 121 from its seat 122. The connection from the valve 122 to the operating cylinder portion above the drive piston 52 is primarily through an axial bore 124 in the mechanism base 80 through a tube 126 interior of the intermediate tube 86 through a hole 128 in the head cap 86 and finally through a radially extending hole 130 to the upper end of a cylinder 90.

As a result of the differential pressure acting on the pistons 52 it and the associated control elements 48 will move at a rather high speed toward their fully withdrawn position. To lessen the shock upon reaching this final position, a shock absorber 132 is mounted in a long counterbore at the top of hole 90. The piston-like shock absorber 132 is positioned against a lower stop 134 by a light spring 135. When the pistons 52 contact the shock absorber piston 132 and moves it upward, water is substantially trapped above the piston 132 by the sealing stop 134. Some of this water is forced past the pistons 132 and produces a retarding force which is dependent upon the shock absorbing piston clearance. The clearance and length of travel is selected so that the forces produced at no time cause a destructive effect upon the moving structure. It should be noted that the pistons 52 cannot be made self-damping since scram time would be greatly increased by the small clearance required for self-damping. Any gases which may be trapped above the shock absorber 132 is forced past the stop 134 into the open area between the stop 134 and the welded plug. 98. This gas is then drawn through an evacuation line 133 to the exterior of the mechanism 50. The various pumping systems have a number of gas-extraction systems (not shown) connected to them to keep gas bubbles from forming in the coolant.

After a piston 52 has reached its upper limit of travel, it is held at this position by external electromagnets 136 and the magnetic valve 1 18 is deenergized so as to conserve the pumping potential of the system. Although two magnets are illustrated, one may be sufficient if the weight of the control rod and drive shaft is not too great. The electromagnets consist of two pole pieces 137 which are attached by welding to a non-magnetic support tube 138 surrounding the mechanism housing and resting on top of the plug 98. As can be seen most clearly in FIG. 12, the inner faces of the pole pieces 137 are machined to obtain a close fit with the head cap 84. The magnetic coil 140 is mounted on a core 142 which is attached by screws or other suitable means to the pole pieces 137. The magnetic path to the control rod pistons 52, which are made of magnetic stainless steel or other suitable material, is completed by plugs 144 made of the same material. The plugs 144 may be threaded into the non-magnetic head cap 84 and may be pressure sealed by welding. The holes 90 should be drilled and surface machined when the plugs are in place to obtain a close fitting magnetic circuit.

When the magnetic coils 140 are energized, the control rod pistons 52 are forced into contact with the wall of the holes 90 and are held in position by friction. As an additional precaution, a ledge 146 is provided in the cylindrical wall of the holes 90 to arrest any slippage of the pistons 52. Scramming of the control elements 48 may be accomplished simply by deenergizing the holding magnets and allowing the associated control rod structure to fall under the influence of gravity. The pressure above the pistons 52 may also be increased to decrease scram time, as will be explained.

The support tube 138 carries all of the magnetic structure associated with each mechanism and is an important feature of the design since it permits access to a failed coil. This would be very difficult if not an impossible task if the magnets 136 were mounted directly on the mechanism housing. This is easily realized when it is pointed out that the length of the mechanisms 50 for a large reactor may exceed 12 feet and that seventy or more such mechanisms may be mounted so closely together that the coils 140 of adjacent mechanisms 50 almost touch. Thus when it becomes necessary to replace a coil, according to this invention, the support tube 138 may be removed from above and a replacement tube with a full complement of magnets 136 may be lowered into place without appreciably disturbing the operation of the pressurized reactor. Replacement of the damaged coils or other defective means can then be made in leisure at a convenient location. In order to accommodate the support tube 138, it is necessary to mount the magnetic valves 118 on the lower extremity of the mechanism 50 as otherwise it would be impossible to remove the support tube 138. The valves 116 of the magnetic valve 118 are supported in housings 148 which are made of a magnetic steel or other suitable material and which fit over non-magnetic steel pressure housing 150 from above and nest on a ledge 151. The pressure housings 150 are enclosed at top by welding to magnetic plugs 153. The housings 148 are provided with lifting knobs 152 at their upper end which fit into slotted counterbores 154 in the lower flanges 156 of the support tube 138. Therefore the valve coils 1 16 will be removed with the holding magnet coils 140 when support tube 138 is withdrawn. A locating pin 158 in the mechanism base 80 cooperates with a guide slot 160 in the support tube 138 to ensure correct alignment between the electrical elements when the tube 138 is brought into position. The pressure housings 150 may be threaded into the lower flange 82 of the mechanisms 50 and may be sealed therein by any suitable means. The valve seats 122 may be secured by a pressure fit and it may be seen that the valve seat 122 has an upwardly directed tubular extension 162 which serves to direct the water flow in a direction parallel to the valve stem and prevents sideward thrust and possible sticking. This last mentioned feature also serves to produce a slight downward force on the valve stem 120 and tip 121 so as to assist in the closing of the valve 1 18 when the coils 1 16 are deenergized. This force may be adjusted by varying the offset in the stem 120 directly above the valve tip 121.

The 300 psi pressure drop exemplified as required to operate the control element pistons 52 is produced by a pump 164 or other suitable means connected as shown in the schematic diagram of FIG. 13. The suction side of the pump 164 is connected through a header arrangement to the low pressure annular region of all the mechanisms for the reactor. The high pressure side of the pump is connected to the outlet pipe 165 of the reactor. A valve 166 is located between the suction side of the pump and the annular low pressure spaces 1 10 which is normally open thus producing a pressure of approximately 1,700 psi in the annular space 110. A high pressure system comprising a high pressure water tank 168, a pressure regulator 169, and a high pressure supply source 170 may be connected to the pump line through a valve 172. The tank 168 may be charged with a pressure approximately 2,200 psi which is higher than that which would exist within the cylinders 90 below the pistons 52. This high pressure system may then be used when desired to decrease the scram time of the control elements 48. Upon receipt of a scram signal, all holding magnets 136 are deenergized and in addition valve 166 is closed and valve 172 is opened. As a result, high pressure water enters the mechanisms 50 where it causes automatic opening of all the magnet valves 118 and the creation of a high pressure in the cylinders 90 above the drive pistons 52 to increase the speed of control element 48 insertion.

By superimposing an AC signal on the DC current of the holding coils 140 which are located closely adjacent the topmost position of the pistons 52 it is possible to obtain an indication that the pistons are actually being fixedly held at their topmost position. The AC coil current will increase considerably when and if a drive piston 52 drops below the magnetic poles 137. Similarly at the lower end of the mechanisms 50 another set of magnetic coils 174 which have pole pieces 176 and are constructed in the same fashion as the upper electromagnets 136 serve to indicate when the drive pistons 52 are fully inserted. The coils 174 are situated just above the lowermost position of the pistons 52 and indicate by utilizing AC current only when a piston has been fully inserted and is thus not closely adjacent the coils 174 and pole piece 176. For this purpose it is necessary that the entire drive shaft 46 be made of a magnetic steel. It is thus possible to ascertain whether any control element 48 is stuck between its fully inserted and fully withdrawn position and also which position it is in.

Before reactor refueling operations can commence, the head assembly 12 with the head penetration adaptors 18 and the mechanisms 50 must be removed, the control elements 48 must be withdrawn from the core into the region of the upper support structure, and the entire upper support structure must be removed to gain access to the fuel assemblies. However, before the control elements 48 are withdrawn primary control of the reactor must be transferred to the chemical shim system. As previously indicated a nulcear reactor such as that disclosed herein is controlled by a combination of a chemical shim system and a control rod system. In the chemical shim system the coolant medium of the reactor includes a neutron absorbing substance such as boron which is relatively homogeneously dispersed throughout the coolant. The boron addition system, or chemical shim system as it is more generally termed, thus serves as an additional means for controlling the reactivity within the reactor. Typically the core of a nuclear reactor may be enriched in fissionable material; for example, containing 3.2 percent fissionable material by weight. Such a core may require a concentration of 1,200 parts per million of natural boron, which on the average includes 20 percent 13 and 80 percent B by weight at the beginning of life. As the core is depleated of enriched fissionable material the boron concentration may be reduced to as littleas 10 ppm.

Where, as in accordance with this invention, the control elements 48 are withdrawn for refueling, it can become necessary to raise the ppm level of boron to 2,500 of ppm and above. Physics calculations for a particular replacement core will dictate the exact amount required. What is essential is that the mode of refueling herein taught not be practiced without ascertaining that the boration system or chemical shim system is fully effective in controlling the reactor core without the presence of the control rods.

Referring to FIG. 14, the upper support structure comprises an upper support plate 22 and upper core plate 24 which are fixedly joined together by columns 23 and in addition comprises drive shaft guiding structures 25. The drive shaft guiding structure 25 essentially consists of a generally square vertical column 2 6 which has a number of guide plates 27 interspersed axially along it. The guide plates 27 are provided with radial slots to permit the passage of the control element gusset plates 44 when it is necessary to insert or remove a control element 48. For purposes of illustrating the removal of the upper structure, two fuel assemblies 36 are shown. The one on the left has the control elements 48 inserted therein while the one on the right has the control element 48 withdrawn into the region of the upper support structure. As may be seen, a special handling tool 180 is provided to lift the control elements 48 from their associated fuel assemblies 36. When the head assembly 12 and its associated structure are removed from the reactor, a large flanged cylinder 182 is placed on the reactor. This cylinder 182 has two horizontal bulk heads 184 which have a number of long tubes 186 (one for each drive location) flared at their lower ends and fixedly supported therein. The inner diameter of the tube 186 is slightly larger than that of the upper guide block lifting fixture 70. When the cylinder 182 is lowered so that its flanges are supported on the upper support plate 22 using the reactor vessel guide studs 187 for accurate location, the guide blocks 70 slip into the flared openings of the tubes 186. Manipulation of the handling too] 180 causes it to couple on the ball extension 78 of the guide block lifting structure 70, and all the control elements 48 associated with a fuel assembly may then be withdrawn. A latching device 188 at the upper end of each tube 186 will hold the lifting fixture 70 and the control elements 48 in their fully withdrawn position until released. After all the control elements 48 are withdrawn, the upper core support structure to which the cylinders 182 are attached by long screws 190 tightened into holes 192 in the upper support plate 22 may be removed from the reactor vessel to a storage space. The removal of the upper support structure and the control elements 48 allows free access to the fuel assemblies 36.

It will therefore be apparent that there has been disclosed a drive mechanism 50 for control elements 48 which is capable of positively and quickly controlling a plurality of control elements 48 which are individually movable and which are designed to be either completely inserted or completely withdrawn from a reactor for control purposes, and which can be withdrawn from the pressure vessel for refueling.

While there have been shown and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired therefore, that the invention be limited to the specific arrangements shown and described. It is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim:

1. The method of refueling a nuclear reactor having water as a coolant which includes a plurality of normally stationary fuel assemblies supported within a pressure vessel between a lower support structure and an upper support structure, and additionally including a plurality of neutron absorbing control elements adapted for telescoping movement within channels formed between fuel elements in selected fuel assemblies, which method comprises:

increasing the amount of homogeneously dispersed neutron absorbing material in the coolant in said pressure vessel to a level sufficient to control the fissionable core with the control elements withdrawn;

withdrawing the control elements from their respective channels into the upper structure;

retaining the control elements within the upper structure;

removing the upper structure with the control elements remaining therein from said pressure vessel whereby the fuel assemblies are exposed, and selectively removing and replacing fuel assemblies.

2. The method of claim 1 wherein the control ele ments have drive shafts attached thereto and the drive shafts extend above the upper support structure when the control elements are withdrawn from the said chan-- nels, which method additionally comprises;

locking the drive shaft in said extending position whereby the control elements are restrained within the upper structure during removal of the upper structure for refueling.

3. The method of refueling a nuclear reactor which includes a plurality of normally stationary fuel assemblies supported within a pressure vessel between a lower support structure and an upper support structure, and additionally including a plurality of neutron absorbing control elements adapted for telescoping movement within channels formed between fuel elements in selected fuel assemblies, said control elements being arranged to be moved by mechanisms located above a head mounted on the pressure vessel, which method comprises:

increasing the amount of homogeneously dispersed neutron absorbing material in the coolant to a level sufiicient to control the fissionable core with the control elements withdrawn;

removing the reactor head and associated mechanisms which adjust the position of control elements in the core while retaining the control elements in the core; withdrawing the control elements from their respective channels into the upper support structure;

removing the upper support structure with the control elements remaining therein to thereby expose the fuel assemblies;

selectively removing and replacing fuel assemblies;

and

repeating the above described steps in reverse order to place the reactor in an operative condition.

4. The method of refueling a nuclear reactor which includes a plurality of normally stationary fuel assemblies supported within a pressure vessel between a lower support structure and an upper support structure, and additionally including a plurality of neutron absorbing control elements adapted for telescoping movement within channels formed between fuel elements in selected fuel assemblies, said control elements being arranged to be moved by mechanisms located above a head mounted on the pressure vessel which method comprises:

increasing the amount of homogeneously dispersed neutron absorbing material in the coolant to a level sufficient to control the fissionable core with the control elements withdrawn;

removing the reactor head and associated mechanisms which adjust the position of control elements in the core while retaining the control elements in the core;

ditional step of mechanically grasping and holding the control elements in the cylinder to securely hold the control elements therein during the reactor refueling process. 

1. The method of refueling a nuclear reactor having water as a coolant which includes a plurality of normally stationary fuel assemblies supported within a pressure vessel between a lower support structure and an upper support structure, and additionally including a plurality of neutron absorbing control elements adapted for telescoping movement within channels formed between fuel elements in selected fuel assemblies, which method comprises: increasing the amount of homogeneously dispersed neutron absorbing material in the coolant in said pressure vessel to a level sufficient to control the fissionable core with the control elements withdrawn; withdrawing the control elements from their respective channels into the upper structure; retaining the control elements within the upper structure; removing the upper structure with the control elements remaining therein from said pressure vessel whereby the fuel assemblies are exposed, and selectively removing and replacing fuel assemblies.
 2. The method of claim 1 wherein the control elements have drive shafts attached thereto and the drive shafts extend above the upper support structure when the control elements are withdrawn from the said channels, which method additionally comprises; locking the drive shaft in said extending position whereby the control elements are restrained within the upper structure during removal of the upper structure for refueling.
 3. The method of refueling a nuclear reactor which includes a plurality of normally stationary fuel assemblies supported within a pressure vessel between a lower support structure and an upper support structure, and additionally including a plurality of neutron absorbing control elements adapted for telescoping movement within channels formed between fuel elements in selected fuel assemblies, said control elements being arranged to be moved by mechanisms located above a head mounted on the pressure vessel, which method comprises: increasing the amount of homogeneously dispersed neutron absorbing material in the coolant to a level sufficient to control the fissionable core with the control elements withdrawn; removing the reactor head and associated mechanisms which adjust the position of control elements in the core while retaining the control elements in the core; withdrawing the control elements from their respective channels into the upper support structure; removing the upper support structure with the control elements remaining therein to thereby expose the fuel assemblies; selectively removing and replacing fuel assemblies; and repeating the above described steps in reverse order to place the reactor in an operative condition.
 4. The method of refueling a nuclear reactor which includes a plurality of normally stationary fuel assemblies supported within a pressure vessel between a lower support structure and an upper support structure, And additionally including a plurality of neutron absorbing control elements adapted for telescoping movement within channels formed between fuel elements in selected fuel assemblies, said control elements being arranged to be moved by mechanisms located above a head mounted on the pressure vessel which method comprises: increasing the amount of homogeneously dispersed neutron absorbing material in the coolant to a level sufficient to control the fissionable core with the control elements withdrawn; removing the reactor head and associated mechanisms which adjust the position of control elements in the core while retaining the control elements in the core; attaching a control element removal tool including a cylinder on the upper support structure; withdrawing the control elements from their respective channels into the upper support structure and retaining them therein; removing the cylinder, control elements and the upper support structure as a unit to thereby expose the fuel assemblies; selectively removing and replacing fuel assemblies and repeating the above described steps in reverse order to place the reactor in an operative condition.
 5. The method according to claim 4 including the additional step of mechanically grasping and holding the control elements in the cylinder to securely hold the control elements therein during the reactor refueling process. 